selftests/bpf: drop custom NULL #define in skb_pkt_end selftest

Now that bpftool generates NULL definition as part of vmlinux.h, drop custom
NULL definition in skb_pkt_end.c.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20210317200510.1354627-3-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/progs/skb_pkt_end.c

@@ -4,7 +4,6 @@
 #include <bpf/bpf_core_read.h>
 #include <bpf/bpf_helpers.h>
 
-#define NULL 0
 #define INLINE __always_inline
 
 #define skb_shorter(skb, len) ((void *)(long)(skb)->data + (len) > (void *)(long)skb->data_end)